The present invention pertains generally to methods and systems for processing organics to produce energy. More specifically, the present invention pertains to methods and systems which gasify organics using hydrothermal treatment and subsequently use the resultant gases to produce energy. The present invention is particularly, but not exclusively, useful as a method and system for gasifying organics in a hydrothermal treatment reactor and subsequently extracting combustible gases from the reactor effluent in a clean form suitable for introduction into an energy producing gas turbine.
The present invention relates generally to processes for the conversion of organics. This conversion can be performed to produce energy, to chemically convert the organics into a less hazardous form, or both. For example, a waste stream that is hazardous due to one or more organic constituents can be processed in accordance with the present invention to produce one or more non-hazardous waste streams. Alternatively, a raw material containing organics can be processed according to the present invention to produce heat which can subsequently be used to produce electricity.
Processes for the conversion of organics can generally be classified as either oxidation processes or reformation processes. In the former, the goal is generally to completely oxidize the organic by reacting it with an excess amount of oxidant. The oxidation reaction is generally exothermic allowing the process to be used to produce energy. In contrast, in reformation processes, the organic is reacted in the absence of an oxidant or with a sub-stoichiometric amount of oxidant to reform or gasify the organic into compounds that are not fully oxidized. One example of a reformation process is the conversion of the large hydrocarbon molecules found in oil deposits into smaller molecules suitable for use in combustion engines. This process is often referred to as gasification because the small molecules produced are generally gaseous under standard atmospheric conditions.
Both oxidation and reformation processes can be performed in aqueous media under supercritical conditions to obtain extremely high reaction rates. For example, U.S. Pat. No. 4,338,199, which issued to Modell on Jul. 6, 1982 discloses an oxidation process in aqueous media, which has come to be known as supercritical water oxidation (xe2x80x9cSCWOxe2x80x9d). As the name SCWO implies, in some implementations of the SCWO process, oxidation occurs essentially entirely at conditions which are supercritical in both temperature ( greater than 374xc2x0 C.) and pressure ( greater than about 3,200 psi or 218 bar). In fact, SCWO has been shown to give rapid and complete oxidation of virtually any organic compound in a matter of seconds at five hundred degrees Celsius to six hundred fifty degrees Celsius (500xc2x0 C.-650xc2x0 C.) and 250 bar. Importantly for the present invention, rapid reaction rates have also been observed for reforming processes such as the gasification of organics under supercritical conditions.
For some feedstocks, rapid reaction rates in either oxidizing or non-oxidizing environments can be achieved at subcritical pressures. For example, U.S. Pat. No. 5,106,513, issued Apr. 21, 1992 to Hong, discloses a conversion process in aqueous media wherein temperatures in the range of six hundred degrees Celsius (600xc2x0 C.) and pressures between 25 bar to 220 bar are used. For purposes of the present disclosure, the various processes describe above for oxidation and reformation in aqueous media are referred to collectively as hydrothermal treatment, if carried out at temperatures between approximately three hundred seventy-four degrees Celsius to eight hundred degrees Celsius (374xc2x0 C.-800xc2x0 C.), and pressures between approximately 25 bar to 1,000 bar.
At typical hydrothermal treatment conditions, densities are in the range of 0.1 g/cc. At these densities, water molecules are considerably farther apart than they are for liquid water under standard conditions. Hydrogen bonding, a short-range phenomenon, is almost entirely disrupted, and the water molecules lose the ordering responsible for many of liquid water""s characteristic properties. In particular, solubility behavior is closer to that of high pressure steam than to liquid water. Smaller polar and nonpolar organic compounds, with relatively high volatility, will exist as vapors at typical SCWO conditions, and hence will be completely miscible with supercritical water. Gases such as N2, O2, and CO2 show similar complete miscibility. Larger organic compounds and polymers will hydrolyze to smaller molecules at typical SCWO conditions, thus resulting in solubilization via chemical reaction. The loss of bulk polarity by the water phase has striking effects on normally water-soluble salts, as well. In particular, because they are no longer readily solvated by water molecules, salts frequently precipitate out as solids which can deposit on process surfaces and cause fouling of heat transfer surfaces or blockage of the process flow.
Theoretically, oxidation under hydrothermal treatment conditions could be used to produce energy. As indicated above, most oxidation reactions are exothermal, and as such can be conducted to produce heat, which can subsequently be used to produce more useful forms of energy such as electricity. Unfortunately, a large amount of energy is expended pressurizing the oxidant prior to introducing the oxidant into the reactor vessel. Thus, only a small amount of net energy is produced, especially when waste streams or other feedstocks having a low thermal value are oxidized.
Unlike oxidation under hydrothermal treatment conditions, gasification does not require the pressurization of an oxidant. Thus, hydrothermal treatment conditions can be used to efficiently gasify an organic and the resultant gases used to produce energy. Unfortunately, for most feedstocks, the effluent exiting the gasification reactor contains particulates, salts and other corrosive species that renders the effluent unsuitable for direct introduction into energy producing devices such as turbines or gas turbines. Further, filtration of solids from the high pressure, high temperature effluent is often impractical, and soluble salts and corrosive species cannot be removed by standard filtration.
In light of the above, it is an object of the present invention to provide a system and method for efficiently processing organics to produce energy. Another object of the present invention is to provide a system and method for processing organics to produce energy that does not expend the large amount of energy necessary to pressurize an oxidant to a pressure suitable for introducing the oxidant into a hydrothermal reactor vessel. It is yet another object of the present invention to provide a system and method for processing organics to produce energy which allows for the efficient processing of organic feedstocks that may contain particulates, salts or other corrosives species. Still another object of the present invention is to provide a system and method for obtaining clean, combustible gases suitable for direct introduction into an energy producing gas turbine from an organic feedstock that may contain particulates, salts or other corrosives species. Another object of the present invention is to provide systems and methods for processing feedstocks to efficiently convert hazardous organic constituents into non-hazardous constituents. Still another object of the present invention is to provide systems and methods for processing feedstocks to produce a clean gas effluent at moderate to high pressures to aid in subsequent gas separation. Yet another object of the present invention is to provide systems and methods for processing feedstocks containing organics to produce energy which are easy to implement, simple to use, and comparatively cost effective.
In accordance with the present invention, a system for processing organic material to produce energy includes a reactor vessel for hydrothermally treating the organic material with water to produce an effluent. Specifically, the organic material is hydrothermally treated in water to gasify the organic material and produce combustible gases in the reactor effluent that can be subsequently used to produce energy. As detailed further below, heat that is generated by the system can be used to preheat the organic material and/or feed water prior to hydrothermal treatment.
A pump is provided to feed the organic material and water into the reactor vessel. In the reactor vessel, the organic material and water are maintained at a temperature between approximately 374xc2x0 C. and approximately 800xc2x0 C. and a pressure above approximately 25 bar. The mixture is maintained at these conditions for a predetermined residence time to gasify at least a portion of the organic material. Exemplary residence times can vary between approximately a few seconds to approximately several minutes. Although the composition of the reaction products that are produced in the reactor vessel depends upon the organic material that is gasified, it is to be appreciated that the effluent exiting the reactor vessel will generally be composed of a hot, gaseous mixture of steam and combustible gases. Further, the gaseous mixture of steam and combustible gases will generally contain particulates, salts and corrosive species, rendering the effluent unsuitable for direct introduction into a turbine or gas turbine.
After hydrothermal treatment, heat can be extracted from the resulting effluent to preheat the feed material. Additionally, combustible gases in the effluent can be extracted for energy production. In detail, the effluent is cooled to a temperature that is sufficient to condense water vapor in the effluent. Preferably, for this purpose, a heat exchanger is used to cool the effluent. The heat extracted from the effluent can then be used to preheat the feed directly (i.e. by passing the feed through the heat exchanger). Alternatively, an exchange fluid can be used to transfer the heat from the effluent to the feed. Specifically, the exchange fluid can receive heat from the effluent in one heat exchanger and then pass the heat to the feed material in another heat exchanger. Importantly, the effluent is cooled sufficiently to condense water vapors that are present in the effluent. It is to be appreciated that the condensed water vapor will scrub any particulates, salts and corrosive species from the remaining gases in the effluent. Typically, cooling the effluent to a temperature below approximately 374xc2x0 C. will be sufficient to condense water vapor. The result is a cooled effluent having a relatively clean gaseous portion and a residual portion containing liquid water, particulates, salts, and other corrosive species.
From the heat exchanger, the cooled effluent is piped to a gas-liquid separator which separates the cooled effluent into a relatively clean gaseous stream and a residual stream. From the separator, the residual stream can be depressurized for further processing or disposal. On the other hand, the clean gaseous stream is expanded using a partial pressure reduction valve and then separated into fractions using a gas separator. For the present invention, at least one fraction of gas from the gas separator is fed into the combustion chamber of a gas turbine, along with air, to produce energy.
In accordance with the present invention, waste heat generated by the gas turbine is used to preheat the feed material before the feed material is introduced into the reactor vessel for hydrothermal treatment. Any remaining waste heat from the gas turbine can be used to produce steam for introduction into a steam turbine to generate energy.